Cryoelectronic receiver front end for mobile radio systems

ABSTRACT

The present invention relates to a novel use of cryoelectronic equipment to implement an extremely sensitive and stable receiver front end for UHF, microwave, and millimeter wave applications. The invention is particularly applicable to base station receivers in mobile radio systems, where the range and capacity of the systems are typically limited by the base station receiver sensitivity.

The present application claims priority under 35 U.S.C. §119 (e) fromcopending U.S. Provisional Application Ser. Nos. 60/002,065 and60/013,942, both entitled “Cryoelectronic Receiver Front End for MobileRadio Systems,” filed Aug. 9, 1995 and Mar. 22, 1996, respectively, eachincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to base station receivers forcommunication applications and specifically to a receiver front end forbase stations used in mobile ratio systems.

BACKGROUND OF THE INVENTION

In terrestrial mobile radio systems of cellular, PCS or other type,geographical areas are subdivided into a number of cells. Thecommunications traffic in each cell is supported by a base station andeach base station has assigned to it a multiplicity of RF carriers. Insuch cellular mobile radio systems that operate at UHF and higherfrequencies, the size of the cells is determined by terrain features(there can not be major obstructions between the mobile station and thebase station), network capacity requirements (the number of users thesystem needs to support), and the base station receiver sensitivity(limited by losses and noise generated in the base station receiverfront end). In such cellular mobile radio systems, cells are calledcapacity cells when their size is determined by traffic requirements,and cells are called coverage cells when their size is determined by thebase station receiver sensitivity and the terrain. Furthermore, adistinction is made between the forward link, which is the radio signaltransmitted from the base station to the mobile station, and the reverselink, which is the radio signal transmitted from the mobile station tothe base station.

In the reverse link, the mobile station typically transmits 10 to 100times less power than the base station transmits in the forward link.Therefore,the received signal strength at the base station is much lowerthan the received signal strength at the mobile station. In situationswhere the base station range is limited by the reverse link signalstrength, the base station is identified as reverse link limited.Likewise, in forward link limited cells the range is limited by thestrength of the signal received at the mobile station.

Mobile radio networks are designed for balanced forward and reverselinks, i.e., equal base station range in both directions. This balanceis based on the assumption that all cells are at full capacity. However,many operational networks are not at full capacity. Under theseconditions, the base station transmitter can be driven harder to providean increased range for the forward link. The cell is then reverse linklimited.

Specialized Mobile Radio (SMR) base stations and rural cellular basestations are typically reverse link limited. In particular, manyexisting cellular base stations are reverse link limited because theywere designed for car phones transmitting at about 8 Watts, while themajority of mobile stations today are battery operated hand-held phones,which transmit at much lower power levels (0.6 Watt in the US and 2Watts in Europe).

Reverse link limitations in specific existing cells due to terrain canbe overcome by increasing the antenna tower height at the base station.More general, construction of additional base stations or repeater sitesis necessary. Both these approaches have major disadvantages: increasingthe height of the receive antennas on the tower is typically notpossible without replacing the entire tower and may violate zoningregulations. Building additional base stations or repeater sites isexpensive and also requires a reassignment of the frequency reusepattern of the network.

In capacity limited cellular networks, additional demand in the numberof users can be met by adding new frequency channels to the existingcell sites if the additional channels are available. In networks whereall channels are in use the only solution is splitting existing cellsinto smaller ones, and correspondingly, adding additional base stationsand reassigning the frequency reuse pattern.

SUMMARY OF THE INVENTION

It is an objective of the present invention to disclose receiver frontend circuitry that can provide significantly increased base stationsensitivity for receiving reverse link signals from mobile stations. Arelated objective is to minimize the noise contributions from cablelosses in the base station receive path which also increases the basestation reverse link sensitivity compared to existing base stations.

Another objective is to reduce the number of base stations in coveragenetworks thereby reducing the installation and maintenance cost of suchnetworks relative to existing cellular mobile radio systems.

Another related objective is to reduce the mobile station transmit powerin coverage or capacity networks by increasing the base station receiversensitivity.

It is a further objective to provide base station receiver front endcircuitry with improved RF filter characteristics to reduceinterference. This feature increases spectrum utilization providingincreased capacity and revenue relative to existing base stations.

Yet another related objective is to operate said receiver front endcircuitry in a thermally stable environment to avoid variationsdegradation in performance, and failure due to ambient temperaturefluctuations.

An additional objective relating to some digital cellular mobile radiosystems is to increase network capacity. These and other objectives areachieved in the present invention which provides a receiver front endfor a base station. The receiver front end includes: (1) filtering meansfor spectrally filtering an RF signal to form a filtered RF signal; (2)amplifying means, in communication with the filtering means, foramplifying the filtered RF signal; and (3) cooling means forcryogenically cooling the filtering means and the amplifying means. Thereceiver front end is substantially adjacent to the antenna to maintainthe insertion loss along a transmission line extending between theantenna and amplifying means at or below a predetermined level. In oneembodiment, the receiver front end is mounted on a structure supportingthe antenna or antenna structure. The filtering and/or amplifying meanscan include circuitry which uses thin film superconducting passiveelectronic components and cryogenically cooled active semiconductorcomponents. The cooling means can be a closed or open cyclerefrigerator. The cooling means can maintain the filtering means andamplifying means at a stable temperature that is independent of thetemperature of the environment external to the cooling means. Thefiltering means, amplifying means, and cooling means will hereinafter bereferred to as the cryoelectronic receiver front end or the receiverfront end. In one embodiment, the cryoelectronic receiver front endconsists at a minimum of a spectral filter and a low noise amplifier,either or both of which can include a superconducting material for thepassive components of the circuit.

To understand the performance advantages of the present invention, it isimportant to relate base station sensitivity with the base station noisefigure. The sensitivity is described as the RF signal power level neededat the receive antenna port to detect a single telephone channel with agiven signal quality. Frequently, in digital mobile radio systems, thissignal quality is described by a frame error rate not exceeding onepercent.

This error rate is a strong function of the signal to noise ratio asmeasured, for example, before the demodulator, and is thus stronglydependent on the noise power. The noise power in turn, is composed ofnoise received by the antenna and noise added by the RF receiver frontend circuitry. The latter can be measured with standard techniques andis typically expressed as a noise figure value. The more noise added bythe receiver, the larger the base station noise figure, the larger thetotal noise power at the demodulator, and the lower the sensitivity ofthe base station.

Cryogenic cooling significantly decreases RF losses in passiveelectronic circuits thereby reducing the thermal noise, also known asJohnson noise. As is also well known, Johnson noise generated in passivecomponents is equal to the component loss when the component is operatedat room temperature, but decreases substantially below the loss valuewhen the component is operated at cryogenic temperature. Additionally,the losses in normal metals decrease with temperature, and the RF lossesof superconducting metals when cooled below the transition temperature,are orders of magnitude lower than that of normal metals. The noisemechanisms intrinsic to a variety of semiconductor transistor designs,such as those used in low noise amplifiers, are also temperaturedependent, and decrease with decreasing temperature. For example, thenoise figure of PHEMT GaAs low noise amplifiers is known tosubstantially decrease when operated at cryogenic temperature.

In addition to the use of cryoelectronic components with extremely lownoise temperature, in the present invention, the RF feed line lossesbetween the receive antenna and the cryoelectronic receiver front endare substantially minimized by locating the cryoelectronic receiverfront end on the antenna mast in close proximity to the receive antennastructure. In cellular base stations it is common practice to locate allbase station electronics including the receiver front end at the base ofthe antenna mast. Depending on the height of the mast, a substantiallength of RF feed line (typically coaxial cable) is used to connect thereceiver front end to the antenna port. This cable causes insertionlosses that directly add to the base station noise figure. In thetypical embodiment of the invention, the cryoelectronic receiver frontend is mounted on the antenna support structure itself. Preferably, theinsertion loss along the transmission line extending between the antennaand the receiver front end is no more than about 1.0 db and morepreferably no more than about 0.5 db.

The noise figure of the cryoelectronic receiver front end of the presentinvention is preferably no more than 1.5 dB, more preferentially no morethan 1.0 dB, and most preferably no more than about 0.7 db. Thiscompares with noise figures in the range of 3 to 8 dB in existing basestations. With respect to base stations sensitivity, this corresponds toa 2 to 7 dB improvement over the existing state of the art. Theconcomitant increase in reverse link range in cellular applications ispreferably at least about 110%, more preferably at least about 120%, andmost preferably at least about 140% of the reverse link range ofconventional systems (i.e., with no tower-mounted cryoelectronicreceiver front end).

The use of superconducting material in the RF spectral filter providesnot only high sensitivity but also improved spectral definition of thecellular band. Ideal bandpass filters have rectangular profiles. Actualfilters have sloping skirts and in-band ripple. The low losses ofsuperconducting material allow the fabrication of very small filtercircuits with steep skirts and low in-band ripple. When used in mobilecellular radio system, such filters allow better use of the availablespectrum, as more channels can be accommodated at the band edges withoutincreased interference from adjacent bands. The small size of thesuperconducting planar filters allows use of more complex filterfunctions to be performed without increasing the size of the mast headcryoelectronic receiver front end and without significant loss insensitivity. For example, combinations of bandpass and bandrejectfilters may be used in base stations where strong out-of-bandinterference signals need to be suppressed. Also, sharper filters can beused to more accurately define specific receive bands or parts thereof.For example, it is customary in the new cellular PCS systems to use60-MHz wide filters. This corresponds to the entire PCS base stationreceive band. In actuality, each licensee only uses a small part of thisspectrum, i.e., either a 15 MHz or a 5 MHz wide band. Superconductingfilters can easily provide the selectivity for these narrower bands withonly a minor increase in noise figure.

Another benefit of the cryoelectronic receiver front end is theincreased spurious free dynamic range compared with existing receiverfront ends. This is the result of the increase in amplifier gain and thedecrease in noise realized through cooling the circuit.

The present invention is applicable to all base station modulation andmultiplexing formats, such as analog or digital modulation, frequency,phase or amplitude modulation, frequency-, time- and code-divisionmultiplexing. The improved sensitivity may be utilized in differentcellular mobile systems in different ways. The benefits include but arenot limited to: balancing of reverse link limited cells; increasing basestation range in coverage networks; increasing cell capacity; betterreception of signals transmitted through buildings and other structures;substantial reduction of degradation in receiver sensitivity caused bythe insertion loss of the transmission line extending from the receiverfront end to the base station because the RF signal is spectrallyfiltered and amplified before transmission along the line; and reducinghandset transmit power levels for safety reasons, for increased talktime, and for better signal quality due to the higher linearity of thetransmit amplifier.

The cryoelectronic receiver front end can readily be applied in cellularmobile radio systems designed to have balanced links. As the transmitpower in the mobile stations is continuously adjusted to the minimumvalue for maintaining a certain reverse link quality, the use of thepresent invention allows mobile stations to operate at substantiallyreduced power levels. This increases the talk time for a given batterysize, and reduces the power levels that users are exposed to.

In mobile radio systems that implement spread spectrum technology, suchas code division multiplex systems, increased sensitivity of the basestation receiver front end as provided with the present invention isknown to significantly increase not only the cell size but also thecapacity.

The filtering means and amplifying means in the receiver front end canbe electronically tunable and/or located on a common substrate. Thetuning means for tuning the filtering means can include a ferroelectricmaterial.

The cooling means can include a cooling device, means for compressing acooling fluid for use in the cooling device, and means for transportingthe cooling fluid between the compressing means and the cooling device.The compressing means can be located near the base of the structuresupporting the antenna, and the cooling device and receiver front endmounted on the upper part of the structure. In this manner, thecompressing means can supply a number of cooling devices with coolingfluid. The cooling fluid can be transported to one or more coolingdevices mounted on the structure by transmitting the cooling fluidthrough a conduit formed by the transmission line extending from thebase station to the receiver front end.

To protect the cooling means and receiver front end from theenvironment, they can be enclosed in a weatherproof enclosure. The inputand output ports in the enclosure for electrical conductors can beprotected from power surges, such as by lightning, by lightningprotection means. The enclosure can form an integral structure with theantenna, particularly with a patch array antenna.

The receiver front end can include switching means for bypassing the RFsignal around the receiver front end in the event of malfunction of thecooling means and/or receiver front end. To activate the switchingmeans, the receiver front end can include monitoring means formonitoring remotely the operation of the various components of thereceiver front end.

For dual diversity reception, a second cooling means for cooling asecond receiver front end can be employed. The antenna is incommunication with the receiver front end and the second antenna withthe second receiver front end. This configuration provides enhancedsystem reliability by providing separate cooling means for the receiverfront ends in each sector.

In another configuration for servicing multiple antennas, a singlecryostat can include a plurality of filtering means and a plurality ofamplifying means. In this configuration, a filtering means andamplifying means are connected with each of a plurality of antennas.

In another embodiment, the present invention provides a method forprocessing a wireless signal transmitted by a mobile station. The methodincludes the steps of: (1) cryogenically cooling a receiver front end inthe base station, with the temperature of cooling preferably being 90%or less of the transition temperature of a superconducting material inthe receiver front end; (2) receiving the signal with the receiver frontend; and (3) transmitting the received signal to the base station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a cryoelectronic receiver front end and

FIG. 1B is a diagram of the receiver front end on an antenna mast inclose proximity to a base station and a compressor;

FIG. 2 is a block diagram of an embodiment of a cryostat and acryoelectronic module;

FIG. 3 is a view of an embodiment of a receiver front end contained in aweatherproof enclosure;

FIG. 4 is a cut away of a cryostat containing multiple cryoelectronicmodules;

FIG. 5 is a block diagram of a cryoelectronic receiver front end with abypass circuit;

FIG. 6 is a block diagram of a cryoelectronic receiver front end systemwhere multiple cryostats are supplied with cooling fluid by a singlecompressor;

FIG. 7 is a block diagram of a fault-tolerant configuration of multiplecryoelectronic receiver front ends supporting a sectorizeddual-diversity antenna system;

FIG. 8A is a simplified perspective view of a dual-antenna system wherethe antenna and cryoelectronic receiver front end are integrated into asingle unit;

FIG. 8B is a view of the integrated antenna and cryoelectronic receiverfront end;

FIG. 9 is a block diagram of a coax cable being used to transportcooling fluid to/from the cryoelectronic receiver front end;

FIG. 10 is a cross-sectional view of the coax cable in FIG. 9 showing aconduit for transporting the cooling fluid;

FIGS. 11A and 11B respectfully depict prior art base station circuitryand a base station circuitry using the cryoelectronic receiver of thepresent invention; and

FIGS. 12A and B depict results of tests using the systems of FIG. 11Aand B.

DETAILED DESCRIPTION

An important aspect of the present invention is the combined use ofhighly conductive materials, particularly superconducting electronicmaterials, and cryocooling devices in a mast mounted RF receiver frontend, to realize substantial benefits in mobile radio systems, based onincreased base station sensitivity combined with high spectralselectivity. An important aspect of the invention lies in the use ofclosed cycle refrigerators, particularly cryopumps, as the cryocoolingdevices. As used herein, a cryopump is a cryogenic refrigeration devicethat entrains molecules on a cooled surface by weak dispersion forces(e.g., entrainment of a gas by cryocondensation, cryosorption, orcryotrapping on a surface that is cooled by a liquid cryogen or amechanical refrigerator).

FIG. 1 shows the cryoelectronic receiver front end of the presentinvention installed in a base station with antenna mast 60 supporting anantenna assembly 56. The cryoelectronic receiver front end consists of amast mounted portion 53 of the receiver front end, a compressor 72, anda conduit 76 for the cooling fluid. The height of the antenna mast ortower varies with application but typically ranges from about 35 toabout 200 feet. As will be appreciated, the height of the towerstructure can vary over a substantially broader range for microcellapplications.

In the shown embodiment, the closed cycle mechanical refrigerator usedfor cooling the electronics to cryogenic temperatures consists of twoparts, one being a compressor 72, and the other being a cold headassembly located inside the mast mounted portion 53. This type ofrefrigeration system is known as a Gifford McMahon refrigerator. In thisconfiguration, the compressor 72 is connected to the receiver front endin the mast mounted portion 53 via a cooling fluid transport conduit 76.It includes two gas lines providing a cooling fluid such as cryogen tothe cold head and a return path back to the compressor. While this isthe preferred embodiment of the refrigerator, other refrigerationsystems may be used that consist of two separate parts, as pulse tubes,Joul-Thompson systems, or that consist of a single integrated part suchas Sterling cycle refrigerators. Preferably, the refrigeration system isrelatively inexpensive, has a heat lift of 5 watts or more at 70K, drawsno more than about 800 to about 1,000 watts of power, and a life inexcess of 1.5 years.

The advantages of using a cryopump such as the Gifford McMahon systemare its high reliability and the fact that the remote compressor can beco-located at the base of the antenna mast with the base station, in anenvironmentally controlled location. Additionally, this configurationallows easy access to the compressor for repair and maintenancefunctions. As will be appreciated, Gifford McMahon systems have a gascompressor remotely located from an expander and connected with theexpander (in the coldfinger) by two flexible, high-pressure hoses. Theexpander includes a cylindrical piston or displacer made from aninsulating material. A regenerator is housed within the displacer whichis in fluid communication with a space on either side of the displacer.The displacer moves up and down within a cylindrical housing. Inoperation, the cooling fluid is introduced into the regenerator via theupper space when an inlet valve is opened and the displacer is at theextreme lower end of the housing. The cooling fluid is cooled as itpasses through the regenerator and the cooled fluid enters into thelower space causing the displacer to rise. The output valve is opened toallow the fluid to expand and cool. The displacer is lowered as thefluid exits through the output valve and the process is repeated. Thereliability of the system results from maintaining a low pressuredifferential across the seals in the displacer (i.e., across the upperand lower ends of the displacer), the use of room temperature input andoutput valves, and the complete removal of entrained compressorlubricating fluid from the cooling fluid.

Again referring to FIG. 1, the base station is connected to the antennaassembly and to the mast mounted portion 53 of the receiver front endvia cable assembly 68. Furthermore, cable assembly 64 connects thereceiver front end 52 to the antenna assembly 56, preferentially withultra-low loss coaxial cables. The quality and length of the coaxialcables 64 is selected in such a way as to minimize the insertion loss.Preferably, the insertion loss along cable assembly 64 is no more thanabout 1.0 dB, more preferably less than about 0.5 db, and mostpreferably no more than about 0.7 db.

Functionally, the cryoelectronic receiver front end is in the receivepath of the base station. Signals from the mobile station are beingcollected by the receive antennas in the antenna assembly, pass throughthe cryoelectronic receiver front end, and then pass to the RF receiverin the base station via coaxial cables 68. In the basic embodiment ofthe invention, the base station RF transmitter is connected directly tothe transmit antennas in the antenna assembly without passing throughthe cryoelectronic receiver front end.

The mast mounted portion 53 of the cryoelectronic receiver front end asshown in FIGS. 1A and 1B, contains the cold head assembly which is acold head 92, and attached to it, cryostat 32. For protection of thesecomponents from environmental effects, the cryoelectronic receiver frontend includes a weatherproof enclosure 52 with connectors (not shown) forthe RF-, power and signal cables and gas lines 76. The enclosure isrelatively compact, typically having a height of no more than about 20inches and, for cylindrically shaped enclosures, a diameter of no morethan about 10 inches. The cryostat is also relatively compact, typicallyhaving a height of no more than about 18 inches and a diameter of nomore than about 8 inches.

The cryostat inside the cryoelectronic receiver front end includes amultiplicity of cryoelectronic modules in correspondence to themultiplicity of receive antennas in the antenna assembly. Typicalwireless base stations have three independent sectors, each with tworeceive antennas for dual diversity, requiring six cryoelectronicmodules. Each module is then connected to one specific receive antennavia a low loss coaxial cable 64. Cable assembly 68 then includestransmit cables connected to the transmit antennas and six receivecables connected directly to the cryoelectronic receiver front end.Other base stations, particularly in coverage networks use omniantennas, in which case two cryoelectronic modules may be needed fordual diversity reception. The benefit of packaging multiplecryoelectronic modules into one cryostat is the reduction in the numberof cold heads needed to operate all modules at cryogenic temperatures.The limitation in the number of circuits that can be packaged in asingle cryostat is the cooling power provided by the cold head, and thesize of the modules. As the complexity and size of the cryoelectronicmodule increases it becomes increasingly more difficult to achieve auniform temperature distribution.

In the basic embodiment of the invention, each cryoelectronic moduleincludes a spectral filter and amplifier as shown in FIG. 2, with thefilter and/or amplifier being fabricated with superconducting materials,such as yttrium barium copper oxide (YBCO) or other such compounds. FIG.2 shows a block diagram of the basic embodiment of the cryostat astaught by the present invention. For simplicity only one cryoelectronicmodule is shown. The cryoelectronic module 20 includes a bandpass filter24 and a low noise amplifier (LNA) 28 contained in a cryostat 32. The RFreceive signal enters the cryostat 32 through a vacuum-tight RFfeedthrough 36 and is guided to the band pass filter 24 via a shortlength of low-loss coaxial cable 40. The filtered signal is connected bya second length 41 of low-loss coaxial cable from the output of thefilter to the input to the LNA 28. The filtered and amplified RF signalis then guided to the output RF feedthrough 48 via coaxial cable 44.

The band pass filter 24 is typically comprised of a superconductingcircuit so as to minimize insertion loss and to provide sharp filterskirts. The frequency band passed by the superconducting filter ispreferably matched to the spectral band licensed by the operator of themobile radio network. For example, in United States PCS systems, thelicensed bands are either 5 MHz or 15 MHz wide. The insertion loss ofsuperconducting filter is preferably no more than about 0.5 db, and morepreferably less than about 0.3 db. The LNA 28 may also be constructed asa superconducting planar hybrid circuit with the semiconductor deviceattached to a superconducting circuit board to improve performance andto further reduce metal losses. In other embodiments, more complexspectral filtering may be incorporated into the cryoelectronic module 20and placed in the cryostat, to provide additional functionality to thebase station receiver. The cryoelectronic module further may includeother components, including but not limited to multi-couplers,diplexers, power dividers and splitters, limiters, mixers, phaseshifters, and/or oscillators. These components may or may not befabricated with superconducting metal as thin film planar circuits. Thevarious circuits may be individually packaged or combined in largerpackages.

The cryoelectronic modules 20 in the cryostat have a preferred operatingtemperature of 90% or less of the superconducting transition temperatureof the superconducting material, more preferentially at or below 77K forYBCO and at or below 90K for TBCCO. It is further stabilized within anarrow temperature range of preferentially less than plus or minus 5Kwith respect to the operating temperature. Being operated in this wayinside the cryostat, the cryoelectronic module exhibits stable RFperformance over a wide range of ambient temperatures.

FIG. 3 shows a pictorial drawing of the mast mounted portion of thecryoelectronic receiver front end 96 in its basic embodiment for asingle sector or omni base station. The cryostat 32 is attached to thecold head 92 and both are mounted in a weatherproof enclosure 52. Theenclosure 52 has a door 100 for assembly, repair and maintenancefunctions. Other embodiments may use cylindrical or other enclosureshapes, with or without doors. The enclosure also includes mountingmeans for mounting it on an antenna mast. RF connectors 104 on theenclosure bottom provide signal input and output ports to the enclosure52. Coaxial cables 106 provide connections between RF connectors 104 andRF ports 36 and 48 in the cryostat (being vacuum tight RF feedthroughs).The cold head cooling gas input and output lines 112 are connected toself-sealing gas connectors 116 on the bottom of the enclosure 52. Gasflow is only possible when the gas lines from the compressor 72 areconnected to the cold head. Power for the cold head and the LNAs insidethe cryostat is applied via power connectors 120 and 122, respectively.In other embodiments a common power connector may be used for the coldhead and the LNAs. In yet another embodiment, power to the LNAs isprovided via one of the coaxial cables 106 and RF connectors 104. Inbase stations with more than one sector, correspondingly more RFconnectors 104, 36 and 48, and cables 106 are used.

In other embodiments the filter and the amplifier, or any of the othercomponents listed above are fabricated not as individual components, andconnected via coaxial cables, but may be fabricated as integratedcircuits on one or more common substrates.

FIG. 4 depicts one basic embodiment of the mounting of thecryoelectronic modules to the coldhead inside the cryostat 32. Thecoldhead 92 has connected to it a cold finger 93 that stands out intothe cryostat. At the top end of cold finger 93 the lowest temperature isachieved. In practice, there is a temperature gradient from the top ofthe adapter 94 to the bottom.

The mounting scheme, shown for single sector dual diversityapplications, uses a mounting platform as the adaptor 94. The adaptor iscomposed of a material having a bulk conductivity of preferably at leastabout 2 watts/cm-K and more preferably at least about 4 watts/cm-K.Preferred materials are metals, such as aluminum, copper and alloysthereof, carbon, and sapphire, and composites thereof. The adaptor 94desirably has a relatively high thermal inertia to slow the rate of heattransfer and therefore the rate of temperature rise in the system duringpower interruptions. Preferably, the thermal energy of the adaptor 94ranges from about 5,000 to about 50,000 joules, more preferably fromabout 10,000 to about 40,000 joules, and most preferably from about20,000 to about 30,000 joules. The ratio of the thermal energy of theadaptor to that of the cold finger preferably ranges from about 200:1 toabout 100:1, and more preferably from about 50:1 to about 10:1. As willbe appreciated, a number of configurations or shapes of the mountingplatform can be employed to mount any two or more cryoelectronic modulesat a desired position on the cold finger. Adaptor 94 is in tight thermalcontact only at the top of the cold finger in order not to distributecooling to the lower parts of the cold finger. Two cryoelectronicmodules 20 are shown mounted on platform 94.

In general, it is not necessary that the amplifiers are operated atexactly the same temperature as the filters.

Amplifiers can be operated at higher temperatures than the filters andstill be within an acceptable degree of insertion loss. Accordingly, theamplifiers can be located at a higher temperature along the coolinggradient existing along the cold finger and/or operated at a highertemperature by placing an thermally insulative material (i.e., amaterial having a thermal conductivity less than that of the adaptor)between the amplifier and the adaptor 94 to slow the rate of heattransfer from the amplifier.

The cryostat also includes radiative head shielding to minimize heatloading of the cryoelectronic module via thermal radiation from thecryostat walls, which are at ambient temperature. In minimizing the heatload to the cold head, it is also preferred that the RF cables to andfrom the cryoelectronic modules are chosen in such a way as to minimizeheat conduction. These techniques are not unique to the invention butare standard practice in cryogenic operated systems.

Diagnostic Monitoring

To monitor the performance of the mast mounted portion of thecryoelectronic receiver front end diagnostic electronics may be includedin the weatherproof enclosure and inside the cryostat. These sensorsignals may be transmitted either analog format or digitally to the basestation via a signal bus and may be used to identify equipment faultsand activate alarm functions. The analog or digital acquisitioncircuitry includes but is not limited to temperature sensors for thefilters and low noise amplifiers, cold head motor power sensor,cryocoolant line pressure sensor, and vacuum sensors, all with thecomparative logic for comparing the measured data to pre-selectedoperating limits. If the data equals or exceeds the operating limit (s),an alarm is produced to alert operating personnel and/or activate an RFby-pass circuit.

RF Bypass Circuit

FIG. 5 shows a block diagram of the RF bypass circuit that can be usedto minimize the impact of a failure of the closed-cycle refrigerator orof one of the cryoelectronic modules. The circuit includes a pair ofsingle pole double throw (SPDT) RF switches 80 that are inserted betweenthe cryoelectronic receiver module 20 and the cables 84 on the one handand the RF ports 36, 48 of the weatherproof enclosure on the other.During normal operation of the cryoelectronic receiver front end, the RFswitches 80 are set in the position that connects the antenna to theinput port 36 of the cryostat and the output port 48 to the basestation. When power to the cryostat is lost, the temperature in thecryostat rises above a predetermined point (usually the criticaltemperature of the superconducting components), the vacuum in thecryostat decreases, one of the LNA fails, or another operating parameteris exceeded, the RF switches 80 automatically switch to their alternatepositions. In this position, the antenna input signal at port 36 isconnected to a by-pass circuit 88 to output port 48 and the basestation. The by-pass circuit can be as simple as a coaxial cable orinclude an amplifier and a spectral filter to maintain the performanceof the receiver front end at a minimally acceptable level.

ELECTRONICALLY TUNABLE FILTER

The spectral filters in the cryoelectronic module may be electronicallytunable. This can for example be implemented with ferroelectricmaterials as disclosed in U.S. Pat. No. 5,472,935, which is incorporatedherein by this reference. Tunable bandpass and bandreject filters may beused separately or in combination. The benefit of electronically tunablefilters is that the frequency band can be shifted remotely withoutaccessing the base station. Electronically tunable band reject filtersmay used to adaptively block strong out of band signals that arereceived by the antenna and degrade the signal to noise ratio of thecommunications channel. To provide such remote control to thecryoelectronic receiver front end, electrical control lines are added tothe filter circuit that pass through the cryostat walls and areconnected to a control circuit inside the weatherproof enclosure. Thecontrol circuit is then connected to the base station via additionalanalog or digital control lines.

Out-of-band interference can result from high RF power levels outsidethe band of interest creating more noise within the band of interest,which decreases the signal to noise ratio of the receiver front end.Such out-of-band signals can leak through the bandpass filter and intothe amplifier. Such out-of-band interference is often due to transmitsignals radiated from antennas in close proximity to the receiver frontend, or to different wireless systems which service the same geographicarea but operate at different frequencies. Bandpass filters are designedto reject signals outside the desired frequency range, but the magnitudeof the rejection, and thus effectiveness for rejecting undesirablesignals, varies as a result of filter design and filter type. It is anobjective of this invention to increase the magnitude of rejection andthus decrease the out-of-band interference by using cryogenically cooledsuperconducting filters in the receiver front end.

MULTIPLE COLD HEADS

FIG. 6 depicts a block diagram of a cryoelectronic receiver front endnetwork, where several cold heads 92 a,b,c, each connected to a singleevacuated vessel 32 a,b,c that houses one or more cryoelectronicmodules, are all supported by a single compressor 72. Each cold head 92a,b,c is connected to a gas manifold 124 via coolant fluid lines 128. Asingle pair of fluid lines 132 connects the manifold 124 to thecompressor 72. For clarity, the RF connections are not shown in thisfigure. The configuration shown allows a single compressor 72 to supportmultiple mast mounted cryoelectronic receiver front ends that may beco-located on the same mast, or separated by distances up to ½ mile fromthe compressor. In applications, where more cryoelectronic modules areneeded than can be integrated into a single cryostat, the use ofmultiple mast mounted cryoelectronic receiver front ends supported by asingle compressor saves installation cost.

Fault Tolerant Dual Diversity Antenna System

FIG. 7 depicts a block diagram of a mast mounted cryoelectronic receiverfront end supporting a base stations with three sectors in afault-tolerant configuration. As shown here six receive antennastructures 56 a-f are used to provide a three-sectored, dual diversityantenna system. Three cryoelectronic receiver front ends 52 a-c areused, with each front end containing two cryoelectronic modules. Coaxcables 64 a-f from the antenna structures 56 a-f are connected to thecryoelectronic receiver front ends 52 a-c in such a manner that the twoantenna structures in each sector (a, b, g) are connected to separatecryoelectronic receiver front ends. In this manner, if a single cryostatfails, two sectors are degraded, but all sectors remain functional.

Integrated Antenna

A further performance improvement of the present invention is possibleby essentially eliminating the cable between the mast mounted cryogenicreceiver front end and the receive antenna.

FIG. 8A is a simplified perspective view of an antenna configuration 136where the antenna 56 is integrated with the mast mounted portion 53 ofthe cryoelectronic receiver front end. This arrangement minimizes thelosses between the antenna and the receiver. Another embodiment of anintegrated antenna is shown in FIG. 8B, where a linear array ofconformal radiating patches 200 is part of the cylindrical weather proofenclosure 96. As will be appreciated, the conformal radiating patchessubstitute for the antenna 56. Such an integrated embodiment is alsodesirable for smart antenna systems, where cryogenically cooled tunablephase shifters are used to electronically steer the antenna beam. Thephase shifters can be mounted inside the cryostat with thecryoelectronic modules. Smart antenna systems are under development nowby a number of companies. Many of these systems require tunable phaseshifters and control electronics co-located with the antenna structure.

Coolant Fluid Transportation

FIG. 9 is a block diagram depicting the use of a coaxial cable fortransporting coolant fluid from the compressor 72 to the cryoelectronicreceiver front end in the mast mounted portion 53. The coaxial cable 68carries the filtered and amplified RF signal from the cryoelectronicreceiver front end to the base station 144.

FIG. 10 is a cross-sectional view of the coaxial cable 68 showing ahollow inner conductive conduit 150 which is partially responsible forcarrying the filtered and amplified RF signal, and is suspended withinthe outer conductor 350 of the cable 68. The cooling fluid istransported within the conduit 150.

In a first embodiment, referring again to FIG. 9, the cooling fluid isintroduced into the inner conduit by using a conductive plug 310 in theconduit 150 “downstream” of the junction 154 to prevent the coolingfluid from flowing down the conduit 150 to the base station 144.Electrically insulative tubing 158 is used to input or output thecooling fluid from the compressor 72 into the conduit 150. Theelectrically insulative tubing prevents the filtered and amplified RFsignal from being removed from the conduit 150 upstream of the station144. Electrically insulating tubing 158 is also used to input or outputthe fluid from the conduit 150 into the cooler (not shown) in thecryoelectronic receiver front end 53. A conductive plug is also used toprevent the fluid from flowing up the conduit past the junction with thetubing and into the amplifier.

In a second embodiment, the fluid is transported in the volume of thecoaxial cable located between the outer conductor 350 and the innerconductor 150. An electrically insulating plug 320 is located“downstream” of the junction 155 to prevent the cooling fluid fromflowing down the coaxial cable 68 to the base station 144.

In a third embodiment, the two previous embodiments are combined suchthat the fluid is transported in both volumes of the coaxial cable. Thishas the advantage that the cooling gas can be delivered and removed fromthe cold head via a single coaxial cable. For example, cooling fluid canbe transported from the compressor to the cold head through the hollowinner conductor (described in the first embodiment), and cooling fluidcan be transported from the cold head to the compressor through thevolume of the coaxial cable located between the outer conductor 350 andthe inner conductor 150 (described in the second embodiment).

In a fourth embodiment, the coaxial cable 68 is enclosed in a hollowstructure (not shown), and the cooling fluid is transported in thevolume located between the inner wall of the hollow structure and theouter wall 350 of the coaxial cable 68.

These embodiments reduce the number of cables that are required betweenthe mast mounted portion of the receiver front end and the compressorand the base station, which in turn will lower the total weight and costof the cables.

Lightning Protection Circuitry

Electromagnetic radiation surges can be introduced to mast mountedelectronics during thunder storms when lightning strikes hit thevicinity of the base station or the base station itself. Mast headelectronics can be provided with lightning protection circuitry forgrounding such electromagnetic radiation surges. Such circuitry may beincluded at all RF input and output ports and all power andsignal/control ports of the weatherproof enclosure. The inclusion ofthese circuits provides a mast mounted receiver front end that isextremely reliable and possesses significant system enhancements overthe prior art.

Experiment

The performances of a conventional base station having the componentsshown in FIG. 11A and of a base station according to the presentinvention having the components shown in FIG. 11B were compared. Theperformances were tested under actual conditions using the same mobilestation. The results of the tests are presented in FIGS. 12A and 12B.The vertical axis of FIGS. 12A and 12B plots the percent residual biterror rate (“RBER”) and the horizontal axis the input signal power ofthe mobile station. The graph shows that for a smaller mobile stationinput signal level, one can achieve the same RBER (i.e., accuracy) withthe base station of FIG. 11B as with the base station of FIG. 11A.Stated another way, for a given mobile station input signal power, thebase station of FIG. 11B provides a superior RBER than the base stationof FIG. 11A. It has been discovered that the base station of FIG. 11Bhas at least a 6 db improvement in bit-per-second receiver sensitivitycompared to that of FIG. 11A. The vertical axis of FIG. 12B plots thetotal noise figure of the base station and the horizontal axis the cableloss. As can be seen from FIG. 12B, the base station using the system ofFIG. 11B has a consistently lower noise figure for a given cable lossthan the base station using the conventional system of FIG. 11A.

Modifications and adaptations of those embodiments will occur to thoseskilled in the art. It is to be expressly understood, however, that suchmodifications and adaptations are within the scope of the presentinvention, as set forth in the appended claims.

What is claimed is:
 1. In a mobile radio system in which the basestation has an antenna and a receiver front end connected to the antennaby a transmission line, comprising: (a) a receiver front end including:(i) a plurality of filtering means for spectrally filtering an RF signalto form at least one filtered RF signal; and (ii) a plurality ofamplifying means, in communication with the at least one filteringmeans, for amplifying the at least one filtered RF signal to form atleast one amplified signal, wherein at least one of the filtering meansand plurality of amplifying means includes a superconductor material;and (b) at least one cryocooling means for cryogenically cooling theplurality of filtering means and the plurality of amplifying means,wherein the receiver front end and cryocooling means are mounted on atower substantially adjacent to the antenna to provide an insertion lossalong a transmission line between the antenna and the receiver front endof no more than about 1.5 dB and wherein a cold finger in thecryocooling means simultaneously engages the plurality of filteringmeans and amplifying means.
 2. The mobile radio system as claimed inclaim 1, wherein the length of the transmission line is sufficient toproduce an insertion loss along the transmission line of no more thanabout 0.5 db.
 3. The mobile radio system as claimed in claim 1, whereinthe length of the transmission line is sufficient to produce a noisefigure for the receiver front end of no more than about 1.0 dB.
 4. Themobile radio system as claimed in claim 1, wherein the plurality offiltering means includes the superconductor material.
 5. The mobileradio system as claimed in claim 1, wherein the cryocooling meansmaintains the plurality of filtering means and amplifying means at asubstantially stable temperature that is substantially independent ofthe temperature of the environment external to the cryocooling means. 6.The mobile radio system as claimed in claim 1, wherein a filtering meansof the plurality of filtering means and an amplifying means of theplurality of amplifying means are located on a common substrate.
 7. Themobile radio system as claimed in claim 1, wherein the plurality offiltering means are electronically tunable.
 8. The system as claimed inclaim 7, wherein the plurality of filtering means comprises a tuningmeans that includes a ferroelectric material.
 9. A system for receivingRF signals, comprising: an antenna for receiving an RF signal; a towerfor elevating the antenna relative to obstructions in the path of the RFsignal; and a receiver front end mounted on an upper portion of thetower in close proximity to the antenna, wherein the receiver front endcomprises: a plurality of superconducting planar filters for spectrallyfiltering the RF signal to form a corresponding plurality of filtered RFsignals; a plurality of amplifiers for amplifying the plurality offiltered RF signals to form a corresponding plurality of amplified RFsignals; and a cryocooler mounted on the tower for cryogenically coolingthe plurality of superconducting planar filters and the plurality ofamplifiers, wherein the cryocooler includes a cold finger forsimultaneously engaging the plurality of superconducting planar filtersand amplifiers.
 10. The system as claimed in claim 9, wherein anamplifier of the plurality of amplifiers comprises a superconductingmaterial.
 11. The system as claimed in claim 9, wherein the plurality ofsuperconducting planar filters and the plurality of amplifiers arecontained within a common cryostat.
 12. The system of claim 9, furthercomprising a transmission line extending between the antenna and thereceiver front end and wherein an insertion loss along the transmissionline is no more than about 1.5 db.
 13. The system of claim 9, whereinthe cryocooler maintains the plurality of superconducting planar filtersand plurality of amplifiers at a substantially stable temperature thatis substantially independent of the temperature of the environmentexternal to the cryocooler.
 14. The system of claim 9, wherein asuperconducting planar filter of the plurality of superconducting planarfilters and an amplifier of the plurality of amplifiers are located on acommon substrate.
 15. The system of claim 9, wherein the plurality ofsuperconducting planar filters are each electronically tunable.
 16. Thesystem of claim 15, wherein the plurality of superconducting planarfilters each include a ferroelectric material.
 17. A method forprocessing a wireless signal transmitted by a mobile station to a basestation, comprising: (a) cryogenically cooling a receiver front end insaid base station with a cryocooler, wherein the receiver front end,cryocooler and an antenna for receiving the wireless signal are mountedon an elevating structure for elevating the antenna relative toobstacles in the path of the wireless signal and wherein the receiverfront end includes a plurality of filters and amplifiers and wherein thecryocooler includes a cold finger that simultaneously engages theplurality of filters and amplifiers; (b) receiving and processing saidsignal with said receiver front end to form a processed signal; and (c)transmitting said processed signal from the receiver front end to saidbase station.
 18. The method as claimed in claim 17, wherein thereceiver front end comprises a superconducting material having asuperconductor transition temperature and in the cryogenically coolingstep, the receiver front end has a temperature that is no more thanabout 90% of the superconductor transition temperature.
 19. The methodof claim 17, further comprising a transmission line extending betweenthe antenna and the receiver front end and wherein an insertion lossassociated with the transmission line is no more than about 1.5 db. 20.The method of claim 17, wherein a filter and an amplifier of theplurality of filters and amplifiers are located on a common substrate.21. The method of claim 17, further comprising the step of altering avoltage applied to a dielectric material in a filter of the plurality offilters and amplifiers to alter a characteristic of the signal duringprocessing of the signal.
 22. A system for receiving signals,comprising: a receiver front end, the receiver front end including aplurality of planar filters for filtering a wireless signal to form aplurality of filtered signals and a corresponding plurality ofamplifiers for amplifying the plurality of filtered signals to form acorresponding plurality of amplified signals; and a cryocooler forcryocooling the receiver front end and for mounting on an antennastructure near an antenna, the cryocooler including a cold finger forengaging simultaneously the plurality of planar filters and theplurality of amplifiers.
 23. The system of claim 22, wherein at leastone of the plurality of planar filters and plurality of amplifiersincludes a superconducting material and the cryocooler is mounted on theantenna structure.
 24. The system of claim 22, wherein a planar filterof the plurality of planar filters and amplifier of the plurality ofamplifiers are located on a common substrate.
 25. The system of claim22, wherein a transmission line extends from the antenna to the receiverfront end and an insertion loss associated with the transmission line isno more than about 1.5 db.
 26. The system of claim 22, wherein theplurality of planar filters and the plurality of amplifiers are locatedin a common cryostat.
 27. The system of claim 22, wherein the cryocooleris to be mounted at the base of the antenna.
 28. The system of claim 22,wherein the antenna structure includes a tower supporting the antennaand the cryocooler is to be mounted on the tower supporting the antenna.29. A method for installing a receiver front end on a structuresupporting an antenna, comprising: providing a receiver front end, thereceiver front end including a plurality of planar filters for filteringa wireless signal to form a plurality of corresponding filtered signalsand a plurality of amplifiers for amplifying the plurality of filteredsignals to form a corresponding plurality of amplified signals; and acryocooler for cryocooling the receiver front end, the cryocoolerincluding a cold finger that engages simultaneously the plurality ofplanar filters and the plurality of amplifiers; mounting the cryocooleron the antenna structure near an antenna; and connecting a transmissionline with the antenna and the receiver front end.
 30. The method ofclaim 27, wherein at least one of the plurality of planar filters andplurality of amplifiers includes a superconducting material.
 31. Themethod of claim 27, wherein a planar filter of the plurality of planarfilters and an amplifier of the plurality of amplifiers are located on acommon substrate.
 32. The method of claim 27, wherein an insertion lossassociated with the transmission line is no more than about 1.5 db. 33.The method of claim 27, wherein the plurality of planar filters and theplurality of planar amplifiers are located in a common cryostat.
 34. Themethod of claim 29, wherein the cryocooler is mounted at the base of theantenna.
 35. The method of claim 29, wherein the antenna structureincludes a tower supporting the antenna and the cryocooler is mounted onthe tower supporting the antenna.